Selective hydrogenation of acetylenes

ABSTRACT

Acetylenes are selectively hydrogenated from the gaseous product from the pyrolysis of substantially sulfur free light hydrocarbons such as ethane and propane. The pyrolysis gas comprises ethylene, butadiene and the acetylenes. The selective hydrogenation occurs by the addition of liquid carbon disulfide into the gas stream in an amount to provide between 5 and 20 parts per million of sulfur in the hydrogenation zone. A catalyst comprising nickel, for example, a nickel-cobalt-chromium sulfide supported catalyst, is employed and the acetylene content of the product is reduced to a level of about 30 to 200 ppm. An ethylene stream can be separated and the acetylene content further reduced in a secondary hydrogenation zone using any suitable hydrogenation catalyst.

United States Patent Ftrench et al.

[451 Sept. 12, 1972 [54] SELECTIVE HYDROGENATION OF ACETYLENES [72] Inventors: Lawrence R. Ffrench; Wilson M.

Skinner, both of Baytown, Tex.

[73] Assignee: Gulf Oil Corporation, Pittsburgh,

[22] Filed: Jan. 22, 1971 [21] Appl. No.: 109,037

[52] US. Cl ..260/677 H, 260/6839 [51] Int. Cl ..C07c 11/00 [58] Field of Search ..260/677 H, 683.9

[56] References Cited UNITED STATES PATENTS 3,301,913 l/l967 Holmes et a1 ..260/677 3,309,307 3/1967 Bryant ..208/144 3,424,809 l/l969 Johnston ..260/677 Primary Examiner-Tobias E. Levow Assistant Examiner-A. P. Demers Attorney-Meyer Neishloss, Deane E. Keith and Richard C. Gaffney [5 7] ABSTRACT Acetylenes are selectively hydrogenated from the gaseous product from the pyrolysis of substantially sulfur free light hydrocarbons such as ethane and propane. The pyrolysis gas comprises ethylene, butadiene and the acetylenes. The selective hydrogenation occurs by the addition of liquid carbon disulfide into the gas stream in an amount to provide between 5 and 20 parts per million of sulfur in the hydrogenation zone. A catalyst comprising nickel, for example, a nickel-cobalt-chromium sulfide supported catalyst, is employed and the acetylene content of the product is reduced to a level of about 30 to 200 ppm. An ethylene stream can be separated and the acetylene content further reduced in a secondary hydrogenation zone using any suitable hydrogenation catalyst.

5 Claims, No Drawings SELECTIVE HYDROGENATION OF ACETYLENES This invention relates to a process for treating an impure gas stream obtained by the pyrolysis of hydrocarbons such as ethane and propane and comprising ethylene, butadiene and acetylenes. In particular, this invention relates to the addition of liquid carbon disulfide to a hydrogenation zone containing a catalyst comprising nickel sulfide to improve the selectivity of the catalyst for the selective hydrogenation of acetylenes in said gas mixture.

Ethylene is one of the largest volume products produced in commercial quantities. Ethylene is normally produced by the pyrolysis of petroleum hydrocarbons such as ethane and propane. When ethylene is produced via this pyrolysis route the entire gas stream from the pyrolysis furnace contains certain impurities which must be removed before the ethylene can be considered to be of acceptable commercial purity. The most objectionable impurity in the pyrolysis stream is the acetylenic materials, such as acetylene itself and methylacetylenes. Other unsaturated hydrocarbons, and in particular butadiene, are also present in the pyrolysis gas stream and must also be removed from the ethylene. The butadiene content of the pyrolysis gas stream is of considerable value as a commercial product itself and thus it is desirable to recover it for sale if possible. Normally, and preferably, the entire pyrolysis gas stream is cooled to condense out a liquid highly unsaturated product termed an aromatic distillate which boils primarily in the naphtha range. The remaining gases require further purification. It is not practical to fractionate the impure pyrolysis gas stream to separate the butadiene fraction from the ethylene since the content of the acetylenic materials is so high, usually on the order of 0.2 weight percent, that considerable polymerization would occur in the fractionation equipment and cause considerable difficulties in plugging of lines, etc. In prior art processes, namely U.S. Pat. No. 3,003,008 to H. W. Fleming et al., it is taught that the selective removal of acetylenes can be achieved without the simultaneous hydrogenation of the butadiene content provided the butene content is sufficiently high. Thus in column 3, lines 1 l-l 7 of the Fleming et al. patent, it is taught that to prevent hydrogenation of butadiene in a gas stream containing no three or four carbon atom monoolefins, an acetylene conversion of only between 85 and 92 percent was obtained. However, when the same gas stream contained about ten percent C monoolefins an acetylene conversion of about 98 percent was achieved without noticeable hydrogenation of butadiene. The

impure pyrolysis gas from the cracking of ethane and propane usually contains from 1.0 to 2.5 mole percent C, monoolefins and thus it would be expected from the teachings of Fleming et al. that the hydrogenation of the acetylenes in the impure gas stream would have to be limited to between 85 and 92 percent in order to prevent the simultaneous hydrogenation of the butadiene content. Unexpectedly, it has been found in accordance with the invention that by the addition of from 5 to parts per million of sulfur in the form of liquid carbon disulfide to a hydrogenation zone containing a hydrogenation catalyst consisting essentially of nickel, cobalt and chromium that the acetylenes can be hydrogenated to a level from 0.10 to 0.25 mole percent to from 30 to 200, usually 30 to ppm, without the simultaneous hydrogenation of substantial proportions of the butadiene content.

The product from the partial hydrogenation can then be treated to separate. an ethylene fraction containing small amounts of acetylene and a higher carbon number fraction from which the butadiene can be separated and sold as a separate product.

The addition of liquid carbon disulfide to insure a sulfur content in the range of 5 to 20 ppm in the hydrogenation zone was unexpected in its effect on the selectivity of the catalyst for the hydrogenation of acetylenes since other prior art is conflicting in its teachings concerning'the effect of sulfur addition. For example, U.S. Pat. No. 3,155,739 to Fleming et al. teaches that the addition of from 1 to 20 ppm of sulfur improves the selectivity of a cobalt sulfide catalyst for the hydrogenation of acetylene in a gas stream containing ethylene, but it is noteworthy that the gas streams treated by Fleming contain only C to C hydrocarbons. Thus, Fleming is not faced with the problem of selectively hydrogenating acetylenes in the simultaneous presence of butadiene. Further, Examples 2 and 6 of Fleming show that inferior results are obtained with a catalyst containing nickel, cobalt and chromium sulfides as compared with Examples 1, 3, 4 and 5 utilizing a catalyst consisting of cobalt .sulfide or cobalt and chromium sulfide. In The Removal of Acetylene from Hydrocarbon Gases by B. E. V. Bowen et al. in J.S.C.I. 69, March, 1950, pages 6569, it is taught on page 68 that the addition of carbon disulfide to a gas obtained from the pyrolysis of refined kerosenes resulted in the immediate breakthrough of acetylene in a hydrogenation'zone employing a nickel-chromium catalyst. Thus it would appear from the teachings of the prior art that the presence of nickel in a hydrogenation catalyst results in inferior results for the selective hydrogenation of acetylenes either with or without the addition of sulfur containing compounds such as carbon disulfide. While it is not certain, it is believed that the poorer results were obtained in the Fleming examples by the fact that in Example 2 too little sulfur was present and in Example 6 too much sulfur was present as an impurity in the gas stream. In the pyrolysis of ethane and propane to produce ethylene, there is little to no sulfur present in the pyrolysis gas stream since the ethane and propane are substantially sulfur-free. By little to not sulfur and substantially sulfur-free is meant a sulfur content on the order of less than 5, usually 1 to 3, ppm. It has now been discovered that when treating such a pyrolysis gas stream and utilizing a catalyst containing nickel sulfide and desiring to selectively hydrogenate acetylenes from admixture with butadiene that the desired selectivity can be obtained by the controlled addition of liquid carbon disulfide to the gas stream entering the hydrogenation zone so as to insure alevel of sulfur content in the hydrogenation zone of from 5 to 20 ppm.

The impure gas stream which is treated by the .process of this invention is obtained by the pyrolysis of light hydrocarbons, such as ethane and propane, which are substantially free of sulfur containing compounds. By substantially free is meant that the cracked gas products have less than 5, usually 3 to 4, ppm of sulfur. A typical analysis of a cracked gas stream suitable for treatment in accordance with this invention is shown in Table I below.

hydrogen is charged downflow to a hydrogenation reactor containing a bed of a catalyst comprising nickel at a space velocity of from 1,000 to 3,5000 volumes of gas per volume of reactor space per hour (preferably a space velocity of 2,500 to 3,500 v/v/hr); a temperature from about 225 to 500 F. (preferably 375 to 450 F.); and an operating pressure of from 50 to 600 psig (preferably 200 to 300 psig). The partial pressure of hydrogen is usually from 30 to 50 psig. Just prior to the entry of the gas into the hydrogenation zone an amount of liquid carbon disulfide is injected into the gas stream so that the total parts per million of sulfur in the gas stream entering the hydrogenation zone is from 5 to 20 ppm, preferably from 6 to 12 ppm. Liquid carbon disulfide has the advantage of being easily metered and pumped into the entering gas stream. In addition, the carbon disulfide does not tend to plug valves as gaseous sulfur compounds might and is conveniently employed in smaller chemical plants in lieu of gaseous sulfur compounds. In actual practice it has been found that when the sulfur addition is within the concentration ranges defined above the temperature increases through the first hydrogenation zone from 45 to 75 F. Without the addition of the liquid carbon disulfide the temperature increase through the first hydrogenation zone was from 80 to 120 F. and sometimes higher. Thus, by the addition of the carbon disulfide in the specified amounts a method has been obtained for controlling the temperature increase through the first hydrogenation zone to an acceptable level of 45 to 75 F. It has been found that by maintaining the temperature increase to within the range indicated that a substantial proportion of the acetylenes in the incoming gas are selectively hydrogenated without the undue loss of valuable butadiene and ethylene constituents in the gas stream. Thus, it has been found that the amount of ethane recycle to the cracking furnaces has been decreased by about two percent, indicating a higher production of ethylene (actually less hydrogenation of ethylene to ethane) since the introduction of the liquid carbon disulfide, and, in addition, the butadiene content of the C fraction from the product has increased from 12 percent to about 35 percent, indicating far less butadiene hydrogenation.

The hydrogenation catalyst which is employed in the first hydrogenation zone comprises a supported nickel containing catalyst. The nickel catalyst can be promoted with one or more of the metals from Groups Vlb and VIII of the Periodic Table. The metals can be present on the support in the form of the metals per se, the oxides, sulfides or mixtures of these forms. The metals from Groups Vlb and VIII include chromium, molybdenum, tungsten, iron, cobalt, palladium, platinum, ruthenium, rhodium, osmium and iridium. The preferred metals for promoting the nickel catalyst are chromium, cobalt, palladium, molybdenum and mixtures thereof. The amount of nickel calculated as the metal can suitably be from 0.5 to 20 percent, preferably 1 to 10 percent, by weight of the catalyst. The weight ratio of nickel as the metal to the total amount of promoting metals from Groups V1 and VIII is suitably from 0.511 to 500:1 but is preferably from 2:1 to 200:1. The most preferred catalysts are the supported sulfur insensitive catalysts containing from 1 to 6 weight percent nickel; from 0.01 to 0.5 weight percent cobalt; from 0.01 to 0.2 weight percent chromium and from 0.01 to 0.1 weight percent palladium.

The supported catalysts can be prepared in a variety of different ways, such as by impregnation of the various metals from a solution of their soluble salts onto the support. For example, a cobalt and chromium promoted nickel catalyst can be prepared by immersing a suitable support material in an aqueous solution of nickel and cobalt nitrate containing some chromic acid anhydride. The catalyst, if desired, can be sulfided by the addition of sulfuric acid to the impregnating solution or by treating the finished catalyst with a sulfiding agent such as hydrogen sulfide. The support material should be a rugged attrition resistant support since polymeric materials are produced in the reaction and the catalyst support must be able to withstand the repeated regenerations to which it is subjected. Suitable support materials include alumina, silica, silica-aluminas, magnesias or mixtures thereof which have the requisite ruggedness of structure. Silica-alumina supports which have been suitably treated as by steaming to reduce their cracking activity are preferred.

The gas products issuing from the first hydrogenation zone contain on the order of 25 to 250, usually 25 to 95, ppm of acetylenes. The total gas product is then fractionated and an ethylene stream is recovered containing on the order of 30 to 200, usually 30 to 80, ppm of acetylene. The acetylene content of the ethylene stream is still too high for commercial pure acceptance. This ethylene stream containing from 30 to 200, usually 30 to 80, ppm of acetylene is normally. passed through a second hydrogenation zone containing a suitable hydrogenation catalyst to reduce the acetylene content to 10 ppm or less, which is an acceptable level for commercially pure ethylene.

The conditions in the second hydrogenation zone include a temperature from to 350 F preferably from to 200 F.; a pressure from 100 to 450 psig, preferably from 370 to 390 psig; a space velocity from 1,000 to 6,000, preferably from 4,700 to 5300, volumes of gas per volume of reactor space per hour; and an amount of hydrogen which is in the range of two to six times the amount necessary to hydrogenate the acetylenes. The partial pressure of hydrogen is from 0.25 to 0.35 psig.

Any suitable hydrogenation catalyst can be utilized in the second hydrogenation zone. Those catalysts which are elective for the hydrogenation of acetylene from monoolefins are, of course, preferred. A suitable group of catalysts which may be employed in the second hydrogenation zone are those which were employed above in the first hydrogenation zone. Other suitable catalysts include those consisting essentially of palladium or platinum on a suitable support such as alumina. The catalyst may or may not be sulfur sensitive since the charge to the second hydrogenation zone may be sulfur-free and no sulfur containing compounds are added. The metals in the catalyst for the second hydrogenation zone are therefore usually in the form of the metal or metal oxide. The metals can be any one or more metals from Groups VIb and VIII of the Periodic Table. The total metals content can be from 0.5 to 30 weight percent of the total catalyst and is preferably from 2 to 15 weight percent. The most preferred catalysts are the nickel-alumina supported catalysts containing from 4 to weight percent nickel promoted with minor amounts (0.01 to 0.1 weight percent) of other metals from Groups VI and VIII, especially chromium, cobalt and palladium.

The catalysts for the second hydrogenation zone may be prepared by any suitable procedures as those defined above for the first hydrogenation zone. The supports may be the same as those for the first zone hydrogenation catalysts.

The invention will be further described with reference to the following experimental work.

A mixture of weight percent ethane and 85 weight percent propane and less than 5 ppm sulfur was pyrolyzed at a temperature from 1450 to 1550 F. The pyrolysis products were cooled to condense out an aromatic distillate fraction and the acid gases (40 ppm of CO and traces of H 5) were removed by scrubbing the total gas stream with dilute monoethanolamine, followed by dilute caustic treatment. The remaining gases had a typical composition as shown in TABLE 11 below.

A gas mixture having the composition shown in TABLE 11 above was passed downflow through a reactor containing a bed of a supported nickel-cobaltchromium sulfide catalyst at a pressure maintained in the range of 225 to 275 psig, a temperature maintained in the range of from 380 to 500 F. and a space velocity of from 2,500 to 3,500 v/v/hr. The catalyst contained about two weight percent nickel, 0.3 weight percent cobalt 0.1 weight percent chromium calculated as the metals on a silica-alumina refractory support. The results of this run are shown in Table III below.

TABLE in Example No. 1 2 3 Reaction Conditions Temperature: F 380-500 402-454 170 Pressure: PSlG 250 250 380 Space Velocity 3290 3290 5250 Products: Mole 2 15.0 15.5 CH, 28.7 27.2 0.2 Acetylenes and Propadiene 0.0060 0.0085 0.0001 Ethylene 28.9 29.6 66.7 Ethane 10.2 10.0 32.8 Propylene 4.8 5.4 0.2 Propane 3.4 3.3 0.1 Butenes 0.5 0.5 1,3-Butadiene 0.5 0.7 Butanes 0.2 0.2 C s and Heavier 7.8 7.6

Referring to Tables 11 and III, it can be seen that the acetylene content was reduced along with the butadiene content. The temperature increase through the catalyst bed was from 380 to 500 F. or F.

EXAMPLE 2 Example 1 was repeated except from 5 to 9 ppm of sulfur asliquid carbon disulfide was metered into the gas stream entering the reactor. The temperature increase through the catalyst bed was from 402 to 454 F or only 52 F. The results of this run are shown in Table 111 above. Referring to Table III, the amount of ethylene hydrogenation was reduced (compare Examples l and 2) and the amount of butadiene hydrogenation was reduced from 37 to l2 percent. The concentration of butadiene in the butene fraction removed in subsequent fractionation was increased from 12 to 34 weight-percent, showing reduced butadiene hydrogenation.

EXAMPLE 3 The product from Example 2 was fractionated at 380 psig and an ethane-ethylene stream containing 85 ppm acetylene was recovered. This stream was passed downflow through a bed of a supported nickel on alumina catalyst (about 6 weight percent nickel) containing minor amounts of chromium and palladium at F., 380 psig and a space velocity of 5,250 v/v/hr. The results of this run are summarized in Table III above and show the product to contain less than 1 ppm acetylene, which is an acceptable commercial level.

Resort may be had to such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

We claim:

1. A process for the selective hydrogenation of acetylenes in a gas stream obtained by the pyrolysis of a substantially sulfur-free light hydrocarbon, said gas stream comprising ethylene; from 0.8 to 1.0 mole percent butadiene and from 0.10 to 0.25 mole percent acetylenes; said process comprising adding to said gas stream an amount of liquid carbon disulfide to insure a level of sulfur in said gas stream of from 5 to 20 ppm and contacting said gas stream containing the carbon disulfide with a hydrogenation catalyst comprising nickel in the presence of hydrogen under hydrogenation conditions including a temperature from 225 to 500 F.; a pressure from 50 to 600 psig; and a space velocity of from 1,000 to 3,500 v/v/hr. and recovering a gas stream containing from 30 to 200 ppm of acetylenes.

2. A process according to claim 1 wherein the catalyst comprises from 0.5 to 20 weight percent nickel on a refractory support.

3. A process according to claim 2 wherein the catalyst contains a promoting amount of at least one other metal from Groups VI and VIII and wherein the weight ratio of nickel to the total amount of promoting metals is from 0.521 to 500:1.

4. A process for the production of improved yields of commercially pure ethylene and butadiene from the products of the pyrolysis of substantially sulfur-free light hydrocarbons, which process comprises cooling said products of pyrolysis to produce a liquid aromatic distillate product and a first gaseous stream comprising ethylene, from 0.8 to 1.0 mole percent butadiene and from 0.10 to 0.25 mole percent acetylenes;

adding to said first gaseous stream an amount of liquid carbon disulfide to insure a sulfur level in said first gaseous stream of from 5 to ppm;

contacting said first gaseous stream with a hydrogenation catalyst comprising nickel in a first hydrogenation zone in the presence of hydrogen under hydrogenation conditions including a temperature from 225 to 500 F.; a pressure from 50 to 600 psig; and a space velocity of from 1,000 to 3,500 v/v/hr.; and

recovering a second gas stream containing from 30 to 200 ppm of acetylenes and without substantial reduction in the butadiene content;

separating from said second gas stream an ethylene fraction containing from 30 to 200 ppm of acetylene;

hydrogenating said ethylene fraction in a second hydrogenation zone with a hydrogenation catalyst under hydrogenation conditions including a temperature from 150 to 350 F.; a pressure from 100 to 450 psig; and a space velocity of from 1,000 to 6,000 v/v/hr. to selectively hydrogenate the acetylene content to a concentration of less than 10 ppm of said ethylene fraction.

5. A process according to claim 4 wherein the catalyst in the first hydrogenation zone is a nickelcobalt-chromium catalyst and the recovered second gas stream contains from 30 to ppm of acetylenes. 

2. A process according to claim 1 wherein the catalyst comprises from 0.5 to 20 weight percent nickel on a refractory support.
 3. A process according to claim 2 wherein the catalyst contains a promoting amount of at least one other metal from Groups VI and VIII and wherein the weight ratio of nickel to the total amount of promoting metals is from 0.5:1 to 500:1.
 4. A process for the production of improved yields of commercially pure ethylene and butadiene from the products of the pyrolysis of substantially sulfur-free light hydrocarbons, which process comprises cooling said products of pyrolysis to produce a liquid aromatic distillate product and a first gaseous stream comprising ethylene, from 0.8 to 1.0 mole percent butadiene and from 0.10 to 0.25 mole percent acetylenes; adding to said first gaseous stream an amount of liquid carbon disulfide to insure a sulfur level in said first gaseous stream of from 5 to 20 ppm; contacting said first gaseous stream with a hydrogenation catalyst comprising nickel in a first hydrogenation zone in the presence of hydrogen under hydrogenation conditions including a temperature from 225* to 500* F.; a pressure from 50 to 600 psig; and a space velocity of from 1,000 to 3,500 v/v/hr.; and recovering a second gas stream containing from 30 to 200 ppm of acetylEnes and without substantial reduction in the butadiene content; separating from said second gas stream an ethylene fraction containing from 30 to 200 ppm of acetylene; hydrogenating said ethylene fraction in a second hydrogenation zone with a hydrogenation catalyst under hydrogenation conditions including a temperature from 150* to 350* F.; a pressure from 100 to 450 psig; and a space velocity of from 1, 000 to 6,000 v/v/hr. to selectively hydrogenate the acetylene content to a concentration of less than 10 ppm of said ethylene fraction.
 5. A process according to claim 4 wherein the catalyst in the first hydrogenation zone is a nickel-cobalt-chromium catalyst and the recovered second gas stream contains from 30 to 80 ppm of acetylenes. 